Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Semiconductor devices perform a wide range of functions such as analog and digital signal processing, sensors, transmitting and receiving electromagnetic signals, controlling electronic devices, power management, and audio/video signal processing. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, diodes, rectifiers, thyristors, and power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontroller, image sensor, application specific integrated circuits (ASIC), power conversion, standard logic, amplifier, clock management, memory, interface circuit, and other signal processing circuits.
An image sensor is a type of semiconductor device that detects and records an image by converting light or electromagnetic radiation into electric signals. An image sensor can be implemented with semiconductor charge-coupled devices (CCD) and active pixel sensors in complementary metal-oxide-semiconductor (CMOS) or N-type metal-oxide-semiconductor (NMOS) technologies with applications in digital cameras, video recorders, medical imaging equipment, night vision equipment, thermal imaging devices, radar, sonar, and other image detecting devices.
Light from the image scene is typically focused onto a flat or planar image sensor surface through one or more optical lenses, e.g., up to four or more lenses. The optical focusing lenses add cost, complexity, and height to the camera package. Even with optical lenses, image quality is often better in the center region and less on the edges of the image sensor. Image sensors are continually driving towards higher resolution, faster focus times, better focus depth, lower profile, and lower cost.
One approach to reducing the number of optical focusing lenses is to make the image sensor with a curved surface. A camera using a curved image sensor is known to have certain performance advantages over one with a flat image sensor, for example in mobile digital camera applications.
FIG. 1 shows semiconductor wafer 10 with base substrate material 12. A plurality of semiconductor die 14 is formed on wafer 10 separated by an inter-die wafer area or saw street 16. Semiconductor die 14 contains an image sensor region, as described above. Semiconductor die 14 has a rectangular or square form-factor with linear side edges 20 and corners 22 and a flat surface. Semiconductor wafer 10 is typically reduced in thickness and singulated through saw street 16 into individual thin semiconductor die 14.
The thin rectangular semiconductor die 14 with a flat surface is placed over a mold or substrate with a curved or concave recess. The surface of the thin semiconductor die 14 is deflected by air pressure or other forces into the concave recess of the mold to form a curved image sensor region 32, as shown in FIGS. 2a and 2b. 
A rectangular form factor of semiconductor die 14 does not readily fit into a curved recess. The additional surface area of the rectangular die form factor over the curved recess creates stress concentration areas. The rectangular image sensor die 14 is subjected to stress when forced into the curved recess of the mold. The stress concentration areas can cause out-of-plane deformation or buckling, as shown in area 34 of image sensor region 32, as shown in FIG. 2a. FIG. 2b shows another example of buckling in area 36 of image sensor region 32. The buckling is dependent on die form factor, die size, die thickness, loading, and ratio of die width to radius of curvature. A rectangular semiconductor die does not readily conform to a concave recess. A smaller die with smaller radius of curvature is more susceptible to buckling. Mechanical instability where compressive forces exceed stiffness of the base substrate material can lead to buckling. In some cases, semiconductor die 14 with image sensor region 32 is susceptible to sensor cracking and other manufacturing defects.